1. Field of the Invention
The present invention relates to a semiconductor device operated by the voltage of a control electrode and more particularly to techniques for improving operating characteristics of the semiconductor device.
2. Description of the Background Art
FIG. 30 is a plan view of a conventional semiconductor device. FIG. 31 is a perspective cross-sectional view thereof taken along the line X--X of FIG. 30. For ease of understanding of the cross-sectional structure, parts of control electrodes and the like are not illustrated in FIG. 31. Reference numeral 1 designates an n.sup.- epitaxial layer; 2 designates a p.sup.+ substrate; 3 designates a protruding portion formed on the surface of the n.sup.- epitaxial layer 1; 4 designates an n.sup.+ diffusion region formed on the upper surface of the protruding portion 3; 5 designates insulating films formed on the side faces of the protruding portion 3 and n.sup.+ diffusion region 4; 6 designates a pair of control electrodes formed on the insulating films 5 on opposite sides of the protruding portion 3 and n.sup.+ diffusion region 4; 7 designates a p.sup.+ diffusion region formed in the n.sup.- epitaxial layer 1 at the end of the n.sup.+ diffusion region 4; 8 designates an insulating film formed on part of the p.sup.+ diffusion region 7, part of the n.sup.+ diffusion region 4 and control electrodes 6; 9 designates an Al-Si electrode formed in contact with the diffusion regions 4 and 7 and isolated from the other portions by the insulating film 8; and 10 designates a metal electrode contacting the p.sup.+ substrate 2.
The operation of the conventional semiconductor device will be described below with reference to FIGS. 32 to 34. As the potential at the control electrodes 6 relative to the electrode 9 is decreased with the potential at the electrode 10 relative to the electrode 9 increased, depletion layers extending from the control electrodes 6 come into contact with each other to generate potential barrier in the protruding portion 3 of the n.sup.- epitaxial layer 1 which lies between the control electrodes 6 as shown in FIG. 32. This prevents electrons from flowing from the electrode 9 toward the electrode 10. Thus current is interrupted.
As the potential at the control electrodes 6 relative to the electrode 9 is increased, the potential barrier disappears and electrons 12 start flowing from the electrode 9 toward the electrode 10. Simultaneously, holes 11 are introduced from the p.sup.+ substrate 2 to cause conductivity modulation in the n.sup.- epitaxial layer 1. As shown in FIG. 33, the introduced holes 11 are joined to the electrons again in the n.sup.- epitaxial layer 1 or n.sup.+ diffusion region 4 or they are absorbed into the p.sup.+ diffusion region 7. Thus the semiconductor device turns on.
As the potential at the control electrodes 6 relative to the electrode 9 is decreased again, the potential barrier is generated again in the protruding portion 3 between the control electrodes 6, so that electronic current tends to stop flowing from the electrode 9 toward the electrode 10. Simultaneously, the introduced holes 11 travel along the surface of the insulating layers 5 in such a manner that they accumulate on the surface thereof to be commutated into the p.sup.+ diffusion region 7, as shown in FIG. 34. Thus the semiconductor device turns off.
The conventional semiconductor device thus constructed has problems to be described below. As the area of the n.sup.+ diffusion region 4 is increased relative to the p.sup.+ diffusion region 7 for reduction in ON-voltage, it takes longer to commutate the holes 11 when the semiconductor device turns off, resulting in reduction in switching speed and increase in switching loss.
On the other hand, as the area of the n.sup.+ diffusion region 4 is decreased relative to the p.sup.+ diffusion region 7, the switching speed and switching loss are improved. However, this causes a large number of holes introduced when the semiconductor device is on to be commutated into the p.sup.+ diffusion region 7, so that the n.sup.- epitaxial layer 1 adjacent the n.sup.+ diffusion region 4 is not sufficiently subjected to the conductivity modulation, resulting in increase in ON-voltage.
The conventional semiconductor device is disadvantageous in that the ON-voltage is in traded-off relation to the switching speed and switching loss so that it is difficult to simultaneously improve both of them.